Engineers are constantly seeking ways to reduce undesirable engine emissions without over reliance upon exhaust after treatment techniques. One strategy is to seek ways to improve performance of fuel injection systems at idle. Over the years, engineers have come to learn that engine emissions can be a significant function of the timing, the number, and the quantity of fuel injections. However, engineers have often found that adjustments in the fuel injection system that result in a reduction of NOx emissions may result in an increase of hydrocarbon and particulate emissions, and vice versa.
For instance, fuel injection systems such as that shown in co-owned U.S. Pat. No. 5,492,098 issued to Hafner et al. on Feb. 20, 1996 use a split fuel injection near top dead center of the engine piston's compression stroke. To begin an injection event, hydraulic pressure acts on a plunger within a fuel injector causing the plunger to advance and pressurize fuel. Further, the pressure on an opening hydraulic surface of a needle valve increases causing the nozzle outlets to open and fuel to be injected into an engine cylinder. As the plunger further advances, a fuel pressurization chamber is briefly in fluid communication with a prime spill port causing the pressure within the fuel pressurization chamber to drop. Thus, the pressure acting on the opening hydraulic surface of the needle valve member is insufficient to hold open the nozzle outlets and continue with the injection event. The pressure briefly drops below valve closing pressure, which allows the needle valve to close. After the plunger advances past the prime spill port, pressure increases within the fuel pressurization chamber and is sufficient to open the nozzle outlets and resume the injection event. The second shot of fuel is generally of a greater quantity than the first shot of fuel.
Although the strategy of the split injection has performed well, there is room for improvement. Although attempts to run a split injection at idle have led to some reduction in undesirable emissions, hydrocarbon and NOx emissions can be further reduced. Moreover, because the two shots of fuel are determined by component geometry rather than electronic control, engineers have difficulty in adjusting the timing and the quantity of each shot to further reduce NOx, hydrocarbon and smoke emissions. Because it is difficult to control the quantity of fuel spilled through the spill port between shots, the quantity of fuel being injected into each engine cylinder varies between fuel injectors and injection events, causing the engine to exhibit unsteady behavior when operating at idle that can be irritating to an operator.
The present invention is directed to overcoming one or more of the problems set forth above.